Pseudonymisation means that personal data is replaced with an identifier (such as a user ID), so that it can no longer be linked directly to a person — but is still reversible in principle, for example by using additional information.
Unlike Anonymisation , pseudonymised data remains subject to the GDPR because a person can still be identified using additional information.
In Web analytics , pseudonymous user IDs are often used to recognise returning visitors without identifying them directly. This data must still be protected and may only be processed on an appropriate legal basis.